Yes, I also thought about dithering, I just wanted to make sure I didn't miss something obvious (like the colour picker trick... Simple, awesome, effective and totally made me drop my jaw since I couldn't come up with it myself ).
Well now that I know for sure that it's the discretization when rendering to the window's back buffer, I can do things again, thanks a lot!
So I guess the banding is visible in darker images, due to the contrast enhancement for darker regions in the human vision? I think I heard something familiar to this in a lecture about visualization...
Last time I heard something about 30bit colordepth it was 2009 when Nvidia released some paper about the Quadro series to support it, thanks for the reminder, too.